Conventional advanced session control protocols, such as the H.323 protocol and session initiation protocol (SIP) make use of so-called “signaling” messages that are messages that enable a terminal to request a connection with another terminal, or similarly messages indicating that a telephone line is busy, or indicating that the called telephone is ringing, or indeed indicating that such and such a telephone is connected to the network and may be reached in such and such a manner.
The H.323 protocol was developed by the International Telecommunication Union standardization sector for telecommunication (ITU-T). It specifies procedures concerning signaling, codec negotiation, and information transport. It is in widespread use by the manufactures of voice equipment and of video conference equipment, and also in several real time Internet applications such as “NetMeeting”.
The SIP was defined by the Internet engineering task force (IETF) in its document RFC 3261. This protocol makes it possible to establish, modify, and terminate multimedia sessions in a network using IP protocol. The SIP protocol also provides procedures for notifying events and sending information outside the context of a session. It is in widespread use for making orders for instant messaging services. Thus, in an SIP environment, there are various types of communication such as requests for setting up sessions and requests that are exchanged without any dialog. In devices that comply with this protocol, the associated functions are implemented by a logic unit referred to as a “user agent” that behaves as a “client” or as a “server”, depending on circumstances.
The disclosure is particularly suitable for infrastructures of the IP multimedia subsystem (IMS) type. The IMS was defined by the Standardization organizations known as the 3rd generation partnership project (3GPP) and the Telecommunications and Internet converged services and protocols for advanced networking (TISPAN). It is a network architecture that was introduced by the 3GPP for mobile networks and then reused by TISPAN for fixed networks. This architecture, which uses SIP protocol, enables multimedia sessions to be set up dynamically and to be controlled between two clients, and it also enables resources to be reserved at the level of the network transporting multimedia streams. By virtue of this architecture, network operators can conveniently implement management policy, deliver a predetermined quality of service, and calculate how much to bill their clients. At present, IMS gives access to services of the following type: telephony, video telephony, presence, and instant messaging, and it also manages the interaction thereof.
When a client registered on a network such as those described above desires to benefit from a multimedia service made available by the network, the client transmits a signaling message to the network or receives one from the network. It sometimes happens that clients have the possibility of using different resources for transporting a given multimedia stream. The network resources concerned by the present disclosure are physical or virtual networks.
In conventional manner, a “physical network” is a set of electromechanical characteristics for transporting data in the form of bits (layer 1 in the open systems interconnection (OSI) model). To make a connection, a client may have various physical interfaces available (e.g. optical fibers, pairs of copper wires, or radio interfaces).
A “virtual network” is constructed on the basis of a physical network and comprises connected communications operating in packet mode. A virtual network makes it possible to obtain specific characteristics, e.g. in terms of quality of service (e.g. data rate, latency, or jitter), in terms of security (e.g. encryption or authentication), or in terms of routing (e.g. a virtual private network (VPN)). A virtual network is conventionally declared on layer 2, 3, or 4 (or intermediate) of the OSI model. When different virtual networks are declared on a given physical network, they are distinguished from one another by identifiers specific to the corresponding OSI layer.
In the technology based on the physical network (X.25, asynchronous transfer mode (ATM), integrated services digital network (ISDN), IP, etc.), it is possible to declare different types of virtual networks. Here are a few examples.
In X.25, the virtual network is identified by a virtual channel identifier (VCI). In ATM, a virtual network is identified by a VCI and also by a virtual path identifier (VPI). X.25 and ATM technologies provide two types of connection: those associated individually with a call referred to as “switched virtual circuits” (SVC), and those set up on a permanent basis between two terminals, referred to as “permanent virtual circuits” (PVC).
In ISDN, the permanent logic link (PLL) service enables user data to be transferred in the form of frames conveyed in the “D-channel”.
In the context of IP, it is possible to use the following:                a virtual local area network (VLAN) in which a dedicated label is added to the data packet headers (layer 2 or layer 3 of the OSI model); or        a multiprotocol label switching tunnel (MPLS) in which, once more, the data packets are labeled (layer 2 of the OSI model); or else        IPSec that makes traffic over IP secure by authentication and/or encryption of data packets (layer 3 of the OSI model).        
The client device having such resources accessible thereto may for example be a fixed terminal or a mobile terminal, or a residential gateway, or indeed a voice gateway of the network operator such as a digital subscriber line access multiplexer (DSLAM) with SIP, i.e. a device that collects digital subscriber line data traffic transiting over some number of telephone lines.
It can then be asked whether it might not be advantageously possible for a client of a multimedia stream who has access to a plurality of network resources to select a particular resource as a function of practical parameters such as: the identity of the sender of a request to set up a multimedia session; and/or the destination of said request; and/or the nature of the stream; and/or the dialed number, etc.
For example, if a voice call is identified as being an emergency call, it may make use of a priority resource.
As another example, a database may define particular categories of clients, for example clients in a category referred to as “secret-services” may be agents of a counter-espionage government service. They could then be allocated secure communications channels when making or receiving calls.
Yet another example might be to select the resource as a function of the nature of a given multimedia stream (audio, video, image, etc.) and of its technical characteristics (data rate, etc.).
In the state of the art, including under SIP protocol, there is no provision in the protocol for informing a client about the network resource that the client may or must select for use. In certain circumstances, recourse is made to palliative solutions that require dedicated equipment to be deployed; such solutions are therefore expensive and complicated to implement.